(A) FIG. 11 shows a configuration of a conventional fuel cell power generation device (conventional technology 1) disclosed in Japanese Patent Laid-Open No. 7-57753. In FIG. 11, a fuel cell body 31 generates DC power by reaction between hydrogen supplied by hydrogen supply means 32 and oxygen in the air supplied by air supply means 33, and a power converter 34 outputs AC power after converting the DC power into the AC power. At an external load command, a power control unit 35 controls a flow rate control unit 36 and the power converter 34, the flow rate control unit 36 controls the flow rate of the hydrogen supply by the hydrogen supply means 32 and the flow rate of the air supply by the air supply means 33 such that the flow rates can be the optimum values, and the power converter 34 controls the amount of electricity output by the fuel cell body 31, thereby successfully controlling the output power. An excess power prevention means 39 including a power detector 37 and an arithmetic unit 38 is provided between the fuel cell body 31 and the power converter 34 so that the amount of electricity can be appropriately suppressed when there is a steep rise in electricity output.
FIG. 12 shows the configuration of the fuel cell power generation device (conventional technology 2) disclosed in Japanese Patent Laid-Open No. 6-325774. In FIG. 12, a fuel cell body 41 generates DC power by reaction between hydrogen supplied by hydrogen supply means 42 and oxygen in the air supplied by air supply means 43, and a power converter 44 outputs AC power after converting the DC power into the AC power. A control device 45 controls a charge/discharge device 46 and the power converter 44, and can control the power output depending on an external load by the discharge from the charge/discharge device 46 or the charge to the charge/discharge device 46 although the amount of electricity from the fuel cell body 41 is constant. In the fuel cell power generation device, since the amount of electricity from the fuel cell body 41 is constant when the power output is controlled depending on a fluctuating external load, the amount of charge or discharge of the charge/discharge device 46 becomes considerably large. Therefore, a large capacity charge/discharge device 46 is required, and the entire device is costly and needs a large installation space.
(B) The configuration of the conventional fuel cell power generation system (conventional technology 3) disclosed in Japanese Patent Laid-Open No. 5-182675 and others is described below by referring to FIG. 13 showing the configuration of the conventional fuel cell power generation system (conventional technology 3).
In FIG. 13, a fuel cell (body) 131 is connected to a load 134 through a battery 132 and an output control means 133 including an inverter.
The operations of the conventional fuel cell power generation system (conventional technology 3) are described below by referring to FIG. 14 which is a graph for explanation of an example of an operation pattern of the conventional fuel cell power generation system (conventional technology 3).
In FIG. 14, the horizontal and vertical axes respectively indicate the time and power, and reference numerals 141 and 142 respectively denote a load power and output power.
The load power 141 is rated power of W8c of the fuel cell body 131 from t2 to t3, and is W8d smaller than the rated power of the fuel cell body 131 from t1 to t2.
On the other hand, in the fuel cell body 131 (FIG. 13), the output control means 133 performs a continuous operation by the rated power W8c from t2 to t3, and performs an intermittent operation by the rated power W8c from t1 to t2 so that the same amount of electricity as the load power 141 can be obtained.
Therefore, the battery 132 (FIG. 13) puts on charge and discharge the redundant power and the insufficient power in the period from t1 to t2.
Since the fuel cell body 131 cannot continue generating power unless a high temperature can be constantly maintained, the energy such as the power for heating the fuel cell body 131 before generating power during power-up is required. Furthermore, since a stopping operation is performed by safely emitting hydrogen remaining in the path while cooling it, the energy such as power, etc. is also required.
Since an intermittent operation is performed in the above mentioned conventional fuel cell power generation system (conventional technology 3), energy is wasted each time power-up and power-down operations are performed.
Although the configuration of the fuel cell power generation system (conventional technology 4) is similar to that of the above mentioned fuel cell power generation system (conventional technology 3), the waste of the conventional technology can be avoided to a certain extent by changing the output power by following the load power as shown in FIG. 15 which is a graph for explanation of an example of an operation pattern of the conventional fuel cell power generation system (conventional technology 4).
In FIG. 15, the horizontal and vertical axes respectively indicate the time and the power. Reference numerals 143 and 144 respectively denote load power and output power. The load power 143 is high in the morning 143b, afternoon 143c, and evening 143d, and is low at midnight 143e and in the early morning 143a. 
The operation of the fuel cell body is controlled by the output power 144 following the load power 143 between the maximum output power W9c and the minimum output power W9d. Since the fuel cell body 1 has a continuously inreasing amount of charge of the battery 132 with excess power when the load power 143 is smaller than the minimum output power W9d at midnight 143e and in the early morning 143a, the operation is stopped.
Thus, the conventional fuel cell power generation system (conventional technology 4) is generally activated and stopped once a day, thereby more successfully reducing the waste energy during the power-up and power-down than the above mentioned fuel cell power generation system (conventional technology 3).
(A) However, in the conventional fuel cell power generation device (conventional technology 1), when an external load command largely changes within a short time, the power control unit 35 has to control the output power by raising and dropping it within a short time, thereby possibly causing the hunting of output power because of the delay of control. As a results there can arise the problem that the operation of the fuel cell power generation device becomes unstable, the efficiency of the device is lowered, and the durability is shortened.
(B) Additionally, there has been the problem that the fuel cell power generation system (conventional technology 4) wastes energy when the operation as shown in FIG. 16 which is a graph for explanation of an example of another operation pattern of the conventional fuel cell power generation system (conventional technology 4) is performed.
To be more practical, in the conventional fuel cell power generation system (conventional technology 4), when there is a temporary rise 145b of load power 145 when the operation is stopped, for example, at midnight 145e or in the early morning 145a, the system is started but stopped soon. Furthermore, in the fuel cell power generation system (conventional technology 4), the stopping process is started but the activating process is soon performed when there is a temporary drop 145d of the load power 145 during the operation, for example, in the afternoon 145c, etc. Thus, energy is wasted in the originally unnecessary activating and stopping operations.